The present invention relates to an aqueous solution for the deposition of tin-zinc alloys, especially an electroplating bath permitting the deposition of tin-zinc alloys from a cyanide-free tin(II)-zinc(II) solution with simple bath management.
Products coated with a tin-zinc alloy by electroplating are characterised by excellent corrosion resistance. Especially the resistance against hydraulic fluid and aqueous salt solutions make products coated in this manner interesting for the automobile industry. Owing to their corrosion resistance and the excellent solderability, products coated with a tin-zinc alloy are also used in the wireless, electro and construction industry.
Electroplating baths common in the past contained tin in the oxidation stage +IV and cyanide ions. However, such electroplating baths have the disadvantage that a higher energy input than for tin(II) ions is necessary for depositing tin(IV) ions. In addition, bath management is faced with the complication that, upon dissolution of the anode, which advantageously also consists of a tin-zinc alloy, the formation of tin(IV) ions must be assisted by formation of a film by means of polarisation. In addition, the thickness and composition of the deposited tin-zinc alloy depends on the current density and thus the geometry of the substrate. In addition, the toxicity of the cyanide ions makes industrial utilisation more difficult.
The latter problem was solved in U.S. Pat. No. 5,378,346 by replacing the cyanide ions with alkali tartrates as complexing agents. However, the problems caused by the use of tin(IV) ions are not solved.
According to EP 0 663 460, an electroplating bath is known which permits the deposition of tin from the stage of the divalent tin so that the necessary energy input may be reduced. The dependence of the rate and composition of the tin-zinc deposition on the current density was also decreased. However, the amphoteric surfactants proposed in this reference lead to a situation where bath management remains critical and must be adjusted and supervised with great accuracy in order to prevent dark and faulty depositions.
It was therefore the object of the present invention to provide an electroplating bath solution for depositing tin-zinc alloys which does not contain cyanide, permits reduced energy consumption during deposition because tin is deposited from a solution containing tin(II) ions, and the bath management of which is not critical over a wide range of freely selectable parameters and permits depositions free of faults and discolouration.
The above object is achieved by an aqueous solution which, in addition to tin(II) and zinc(II) ions, contains aliphatic carboxylic acids and/or alkali salts thereof as complexing agents as well as a mixture of anionic and non-ionic surfactants as grain refining agents.